Swedish Patent Application No. 7801877-7 discloses that pulps of high quality can be manufactured at lower energy consumptions than had been possible previously by using a material having a fiber concentration lower than the usual conventional concentration. When the fiber concentration between a pair of refining discs is lowered to levels below about 15 percent, the fiber material can be refined to obtain a desired pulp quality without consuming the amount of energy normally required for refining material having higher fiber concentrations.
It is difficult, however, to utilize material having fiber concentrations lower than about 15 percent in a conventional refiner, due to the difficulty in obtaining a sufficient retention time of such material and, hence, treatment of the fiber material between the discs. At low fiber concentrations, the fiber network offers too little resistance to the centrifugal force which urges the material towards the periphery of the discs. By moving the discs closer to each other and thereby reducing the gap therebetween, it is possible to increase the resistance of the fiber material to movement so as to increase the retention time of the fiber material in the gap. However, at these low fiber concentrations, the gap must be reduced to such a small size that the possibility of fiber damage increases substantially.
As disclosed in the Swedish Patent Application No. 7801877-7, a controlled amount of water, preferably waste-water from the mill, is continuously supplied to a refiner housing surrounding the refining discs, so that the housing is filled with a fiber suspension of easily pumpable concentration, preferably 1-6 percent. The fiber suspension in the housing forms a barrier adjacent to the discharge opening of the gap, thereby making it difficult for the fiber material to pass out of the gap, i.e., the fiber material is braked and transported more slowly through the gap.
The rotation of one or both of the refining discs in a housing filled with a fiber suspension is disadvantageous because it causes energy to be consumed as the result of (i) the friction between the rotating refining disc or discs and the fiber suspension and (ii) the turbulence in the suspension. This no-load energy is lost energy and of no benefit at all to the refining operation. Therefore, it should be reduced to a minimum in order to save energy.